The present invention relates to an optical disc reproducing apparatus which reproduces data from an optical disc such as a CD or a DVD, and also to a tracking servo mechanism for an optical disc which performs a tracking servo control on an optical disc.
In a tracking servo mechanism for an optical disc of the three-beam method, as shown in FIG. 5, in addition to a spot M of a main beam for reading data from an optical disc, a spot S1 of a front sub-beam which impinges in front of the main beam with respect to the track direction of the optical disc, and a spot S2 of a rear sub-beam which impinges in rear of the main beam with respect to the track direction of the optical disc irradiate an optical disc. The difference between output signals of optical sensors which receive respectively reflected light beams of the front and rear sub-beams from the optical disc is used as a tracking error signal.
When the spot M of the main beam is positioned in the center of a track T of an optical disc as shown in FIG. 5B, the intensities of the reflected light beams of the spots S1 and S2 of the front and rear sub-beams from the optical disc are equal to each other. By contrast, when the spot M of the main beam is not positioned in the center of the track T of the optical disc as shown in FIGS. 5A and 5C, the intensities of the reflected light beams of the spots S1 and S2 of the front and rear sub-beams from the optical disc are differentiated from each other. A tracking servo control is performed by using this phenomenon.
In an optical disc, there is a possibility that a defective area due to a scratch or dirt exists. In such a defective area, the intensity of a reflected light beam from the optical disc may have an unexpected value to cause track slipping (the spot of the main beam is moved to another track), and hence the tracking servo control is stopped. A specific process relating to the above will be described.
As shown in FIG. 2, four output signals A, B, C, and D of a four-split photodiode 11 which receives a reflected light beam of the main beam from the optical disc are added together by an adder 21, and a resulting signal is passed through a low-pass filter 22, whereby a full adding signal ASO is produced. The value of the full adding signal ASO varies in accordance with the relationship between a defective area N of the optical disc and the center position of the spot of the main beam, as shown in, for example, FIG. 6. In this example, when the level of the full adding signal ASO becomes higher than a threshold th, therefore, it is judged that the spot enters the defective area N, and the tracking servo control is turned OFF. When the level of the full adding signal ASO then becomes lower than the threshold th, it is judged that the spot exits the defective area N, and the tracking servo control is turned ON.
In the above-mentioned process, as shown in FIG. 7, the tracking servo control is resumed during a period when the spot S2 of the rear sub-beam is positioned in the defective area N of the optical disc. Therefore, there remains the possibility of occurrence of track slipping.
The Unexamined Japanese Patent Application Publication No. Hei 8-55351 discloses a technique the objective of which is to prevent track slipping due to a defective area from occurring. In the technique, the tracking servo control remains to be turned ON also during a period when a beam spot is positioned in a defective area, and therefore there is a possibility that track slipping cannot be prevented from occurring.